1. Field of the Invention
The present invention relates to electronic semiconductor devices, and, more particularly, to high frequency field effect power transistors.
2. Description of the Related Art
Microwave power transistors typically are metal semiconductor field effect transistors (MESFETs) with recessed gates and n type gallium arsenide (GaAs) semiconductor channels. A recessed gate lengthens the surface leakage path from gate to drain and permits the channel electrons to be slightly removed from the surface and attendant surface state scattering. The cut-off frequency (f.sub.t) of conventional GaAs MESFETs depends on the gate length roughly as ##EQU1## where G.sub.mO is the intrinsic transconductance, C.sub.gs is the gate-source capacitance, v is the average carrier drift velocity in the channel, L is the effective channel length (gate length), and .tau. is the average transit time for a carrier traversing the channel. Thus reducing gate length increases f.sub.t, and millimeter-wave GaAs MESFETs for 60 GHz operation typically have 0.25 .mu.m long gates. But to limit short channel effects, the channel thickness (usually denoted by a) must be much less than one third of the gate length (i.e., L&gt;&gt;3a); see H. Daembkes et al, Improved Short-Channel GaAs MESFET's by Use of Higher Doping Concentration, 31 IEEE Tr.Elec.Dev. 1032 (1984). Thus, the channel thickness is decreased with decreasing gate length. However, device power depends upon the channel current, so the doping concentration in the channel must be increased to compensate for the thinner channel. However, the resultant high doping concentrations lower the breakdown voltage (V.sub.b) and power density per unit gate width. And because of the nonuniform field distribution in the channel, the breakdown voltage is also proportional to the total charge in the depletion layer under the gate at pinch-off; see W. Frensley, Power-Limiting Breakdown Effects in GaAs MESFET's, 28 IEEE Tr.Elec.Dev. 962 (1982) which demonstrates a good approximation for the breakdown voltage is: ##EQU2## where N(y) is the doping density as a function of depth in the channel and the integration is over the channel thickness. But total charge in the depletion layer at pinch-off is proportional to the maximum current (presuming no charge accumulation): EQU .function.N(y)dy.apprxeq.I.sub.max.
Thus I.sub.max is inversely proportional to V.sub.b : ##EQU3## and the output power per unit gate width of a MESFET is: ##EQU4## Therefore, the power of a standard MESFET is limited by the relationship between V.sub.b and I.sub.max.
The output power of GaAs FETs can be improved by using insulated gates to increase either V.sub.b or I.sub.max without decreasing the other. Such metal insulator semiconductor FETs are called MISFETs. Breakdown occurs at the drain-side edge of the gate where the electric field is greatest and involves avalanche multiplication, and for MISFETs the breakdown voltage in this region is very high due to the wide bandgap of the insulator. However, fabrication of MISFETs from GaAs and other III-V compound semiconductors has proved difficult due to the large interface state densities at the insulator-channel interface; see, for example, T. Mimura et al, GaAs Microwave MOSFET's, 25 IEEE Tr.Elec.Dev. 573 (1978) which grows native oxides on GaAs to form MOSFETs; and B. Pruniaux et al, A Semi-Insulated Gate Gallium-Arsenide Field-Effect Transistor, 19 IEEE Tr.Elec.Dev. 672 (1972) which ion implants argon into GaAs to form a semi-insulating layer to act as a gate insulator. These approaches have not been fruitful.
A different approach to III-V MISFETs appears in J. Baranrd et al, Double Heterostructure Ga.sub.0.47 In.sub.0.53 As MESFETs with Submicron Gates, 1 IEEE Elec.Dev.Lett. 174 (1980) which uses an undoped AlInAs gate "insulator" on an n type GaInAs channel together with an undoped AlInAs barrier under the channel to confine electrons to the channel. The AlInAs insulator forms a lattice-matched heterojunction with the GaInAs channel and consequently has low interface state densities; but this device has insufficient power handling and the donors in the GaInAs channel scatter the conduction electrons. Also see B. Kim et al, Microwave Power GaAs MISFET's with Undoped AlGaAs as An Insulator, 5 IEEE Elec.Dev.Lett. 494 (1984) which uses an undoped Al.sub.x Ga.sub.1-x As gate insulator on a GaAs channel doped n type to a carrier concentration of about 3.5.times.10.sup.17 /cm.sup.3.
Another approach is the high electron mobility transistor (HEMT) or modulation doped field effect transistor (MODFET) which typically has an n type Al.sub.x Ga.sub.1-x As gate insulator layer epitaxially grown on and forming a heterojunction with an undoped GaAs channel; the donated electrons migrate from the Al.sub.x Ga.sub.1-x As into the GaAs due to the conduction band discontinuity and form a two-dimensional electron gas (2DEG) at the interface. The 2DEG provides very high mobility electrons but little power handling capability due to low current levels and a low breakdown voltage of n type Al.sub.x Ga.sub.1-x As. Donors in the Al.sub.x Ga.sub.1-x As gate insulator screen the gate voltage, and forward bias leads to parallel conduction in the gate insulator and low transconductance. Enhancements such as two n type Al.sub.x Ga.sub.1-x As layers, one on either side of the undoped GaAs channel, to provide two interfaces each with a 2DEG do not solve the problems; see S. Judaprawira et al, Modulation-doped MBE GaAs/nAl.sub.x Ga.sub.1-x As MESFETs, 2 IEEE Elec.Dev.Lett. 14 (1981). Similarly, Delagebeaudeuf et al, U.S. Pat. No. 4,455,564, combines a MESFET with a HEMT to have a metal gate on a thin heavily doped GaAs channel which is on a thin undoped GaAs second channel which in turn is on a heavily doped Al.sub.x Ga.sub.1-x As layer to form a 2DEG in the second channel does not solve the problems. And the heterostructure insulated gate field effect transistor (HIGFET), which has a gate on an undoped Al.sub.x Ga.sub.1-x As insulator which is on an undoped GaAs channel and relies on gate bias to induce a 2DEG, does not have high power handling capability and is typically a lower power, digital device; see N. Cirillo et al, Complementary Heterostructure Insulated Gate Field Effect Transistors (HIGFETs), 1985 IEEE IEDM Digest. p. 317.
Thus there is a problem to provide a FET structure with high power handling capablities, high breakdown voltage, and high transconductance at microwave frequencies.